Unusual wave activity in the Pc 1-2 frequency band (0.1-5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Hallowe'en storm on October 29, 2003, intense broadband activity between ∼0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O+ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4-5.9, 10-15° south of the magnetic equator. We infer that this wave event may have been generated on lower L shells and propagated obliquely to Cluster's location. Consistent with other recent observations, anisotropic plasma sheet/ring current proton distributions appeared to be a necessary condition for occurrence of waves during both passes, but was not always a sufficient condition. The transverse waves of November 22 occurred in regions which also contained greatly increased fluxes of cool ions (E<1 keV). On both days, Cluster observed features not previously reported, and we note that the purely compressional nature of the October 29 events was not anticipated in previous theoretical studies. The fact that these unusually polarized waves occurred in association with very intense geomagnetic storms suggests that they are likely to be extremely rare.
Bibliographical noteFunding Information:
We thank the two referees for their constructive and detailed comments, J.F. Fennell of the Aerospace Corporation and Q. Zong of Boston University for discussion of energetic ion fluxes, Jon-Erik Hokenson of Augsburg College for his help in processing the Cluster FGM data, and John Dombeck of the University of Minnesota for preparing the Poynting vector plots. Provisional D st data were obtained from the World Data Center for Geomagnetism, Kyoto. The involvement of MJE in the Cluster project was supported by a grant from the German Fulbright Commission and by sabbatical leave funds from Augsburg College. The work of KHF and KHG was financially supported by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) and the German Bundesministerium für Bildung und Forschung under contract 50OC0103. Work at the University of California-Berkeley was supported by NASA grants NNG04GF23G and NAG-11416, work at the University of Minnesota was supported by NASA grants NAG5-11944 and NAG5-11124, and work at Princeton University was performed under the auspices of NASA grants W-19880 and NNH04AB23I and National Science Foundation Grant ATM-0218847.
- MHD waves
- Magnetic pulsations
- Magnetic storms
- Plasma waves and instabilities